The invention relates to a method for producing a capping layer composed of silicon oxide on a coating of a mirror, said coating reflecting EUV radiation. The invention likewise relates to a mirror comprising a capping layer composed of silicon oxide, and to an EUV lithography apparatus comprising such a mirror. The mirror can also be used in a different optical device than an EUV lithography apparatus, e.g. in an EUV mask metrology system.
Projection exposure apparatuses for microlithography serve for producing microstructured components using a photolithographic method. In this case, a structure-bearing mask, the so-called reticle, is imaged onto a photosensitive layer with the aid of a projection optical unit. The minimum feature size that can be imaged with the aid of such a projection optical unit is determined by the wavelength of the imaging light used. The smaller the wavelength of the imaging light used, the smaller the structures that can be imaged with the aid of the projection optical unit. Imaging light having the wavelength of 193 nm or—in so-called EUV lithography apparatuses—imaging light having a wavelength in the range of the extreme ultraviolet (EUV), i.e. 5 nm-30 nm, is principally used nowadays. Reflective optical elements (EUV mirrors) are exclusively used in EUV lithography apparatuses since no optical materials having a sufficiently high transmission at these wavelengths are known.
An EUV mirror for such an EUV lithography apparatus comprises a substrate and a reflective coating having a plurality of layers, said reflective coating being applied to the substrate. Such a multilayer coating generally consists of alternating layers composed of materials having high and low refractive indices, e.g. alternating layers composed of molybdenum and silicon, the layer thicknesses of which are coordinated with one another such that the coating fulfils its optical function and a high reflectivity is ensured. The reflective multilayer coating typically has a capping layer in order to protect the underlying layers against oxidation. Said capping layer can consist of a metallic material, e.g. of ruthenium, rhodium or palladium.
EP 1 065 568 A2 discloses using carbides as materials for the capping layer, e.g. boron carbide (B4C) or silicon carbide (SiC). Nitrides, for example silicon nitride (Si3N4) or titanium nitride (TiN), are also specified there as materials for the capping layer. Analogously, US 2006/0066940 A1 describes an EUV mirror comprising a capping layer system, wherein alongside silicon nitride (Si3N4) boron nitride (BN), too, and alongside boron carbide (B4C) molybdenum carbide (MoC) and silicon dioxide (SiO2), too, are proposed as materials for the capping layer system.
It is also known inter alia from the article “Top Layer Oxidation in Mo/Si Multilayer X-Ray Mirror” by Khanh Nguyen et al., Mat. Res. Soc. Symp. Proc. Vol. 306, 1993 (Materials Research Society) that in the case of an Mo/Si multilayer coating in which a layer of silicon forms the terminating or capping layer, a thin film of silicon dioxide (SiO2) forms in ambient air, the thickness thereof typically being between approximately 10 Angstroms and 20 Angstroms.
US 2010/0190113 A1 describes a mirror comprising a capping layer system having a plurality of layers. The topmost layer of the capping layer system can be formed from silicon dioxide, for example, which is applied using a reactive sputtering process.
During the operation of an EUV mirror with such a capping layer composed of silicon dioxide and produced by sputtering in an EUV lithography apparatus, it has been found that during hydrogen cleaning in which the surface of the capping layer is brought into contact with activated hydrogen in order to remove contaminations such as carbon or possibly tin from said layer, a partial conversion or reduction of the silicon dioxide takes place, during which silanes are possibly formed, which has a disadvantageous effect on the reflectivity and/or the uniformity of the reflectivity.